A. Field of the Invention
The present invention is related to a circuit for tuning the resonant frequency of a resonator.
B. Description of the Related Art
Electrically tunable resonators are used in high-frequency technology as a part of filters and voltage-controlled oscillators (VCO). In a filter, tuning the resonator resonant frequency affects the center frequency of the pass band or stop band of the filter, for example, allowing a tunable filter to replace several filters having a fixed center frequency. Electrical tuning may also be directed to the bandwidth of a filter. Tunable filters are beneficial in radio devices which operate in several data transfer channels and incorporate printed circuit boards which must be small and inexpensive to manufacture. A good example of a target application of a tunable filter is a modern, small-sized mobile telephone.
In a voltage-controlled oscillator (VCO), resonator tuning affects the output frequency of the oscillator. Mobile telephones also are the most important target applications of voltage-controlled oscillators. The burst mode transmissions of digital telephones based on time-division technology and the system solutions of the radio frequency part of a telephone in which a VCO operates in a wide range of frequencies place stringent requirements on resonators as far as noise resistance and the frequency tuning range are concerned.
Known procedures for electrically tuning the resonant frequency of transmission line resonators are mainly based on tuning the capacitive load at the non-grounded high impedance end of the resonator. The tuning circuit may consist of a voltage-controlled tuning circuit, for example, which consists of one or more capacitance diodes which are galvanically connected in parallel with the center conductor of the resonator, between the high impedance end and ground. A capacitance diode functions as a tunable capacitance. For example, in a coaxial resonator, a capacitance diode can be placed between the upper edge of the resonator's loaded end, or hole, and the grounded upper surface. A similar tuning circuit functions in all transmission line resonators. A wide tuning range is achieved by tuning the capacitance.
However, there are problems related to capacitance tuning. The tuning circuit significantly increases resonator loss. This results in undesirable pass band attenuation, for example, in a filter made up of resonators. Furthermore, the components commonly used in the circuits, especially capacitance diodes, can not withstand the high voltages and power produced by the strong electric field at the open end of the resonator. Overloading of the components can also be detected as unstable resonator operation. Attempts have been made to eliminate the problems created by the tuning circuit components in coaxial resonators, for example, by placing the capacitance diode in the resonator hole, where the strength of the field is near zero. However, this solution causes manufacturing problems and, in practice, it is suitable only for coaxial resonators.
One way of realizing electrical tuning of a resonator is to place another resonator, or side resonator, next to the resonator being tuned, or the main resonator, which side resonator has a resonant frequency which is suitably higher or lower than the resonant frequency of the main resonator. One end of the side resonator has a controllable switch by which the resonator can be short circuited to ground. The controllable switch may be a capacitance diode, for example. Tuning of the resonant frequency of the main resonator by means of the side resonator is based on a connection between the resonators. The principle is that when the switch is open, the side resonator functions as a half wave resonator, whereupon its resonant frequency is so distant from the resonant frequency of the main resonator that no tuning effect is realized between the resonators. When the switch is closed, the side resonator becomes a quarter wave resonator which affects the resonant frequency of the main resonator. This method of tuning eliminates the effects of large voltages and radio frequency power on the tuning circuit, particularly on the capacitance diode. The method is mainly suitable for tuning dielectric resonators, especially strip line resonators realized on the surface of a dielectric component.
One way of tuning the resonant frequency of a helix resonator is to short circuit windings of the resonator coil with a PIN-diode, for example, causing the resonant frequency of the resonator to increase. Correspondingly, the short circuit can be removed by making the PIN-diode non-conductive, causing the resonant frequency to decrease. The tuning range is determined by the number of "short circuits" installed on the helix coil. The current flowing through the diodes, which is proportional to the voltage difference of the short circuited windings, is small compared to the current flowing between the open end of the resonator presented in conjunction with capacitance tuning and ground. This construction eliminates problems related to the tuning circuit's ability to withstand power and voltage. The problems with this method of tuning are related to its realization. The smaller the resonator is, the more difficult it is to solder "short circuits" onto the resonator coil. This method of tuning is best suited for helix antennas.